Atomic Structure (Ch1)

Question 1

State the three sub atomic particles

Answer 1

• proton
• electron
• neutron

Question 2

State the relative mass and charge of a proton

Answer 2

• 1

- +1

Question 3

State the relative mass and charge of an electron

Answer 3

• 0

- -1

Question 4

State the relative mass and charge of a neutron

Answer 4

• 1

- 0

Question 5

Why do we assume there’s an equal number of protons and electrons in an atom?

Answer 5

• because they have no charge

- so their number of positive and negative charges must be equal

Question 6

Why don’t protons in the nucleus repel?

Answer 6

• strong nuclear force holds together protons and neutrons
• this is far stronger than the electrostatic repulsions
• therefore it overcomes the repulsion between protons

Question 7

Define atomic number

Answer 7

• number of protons in the nucleus

Question 8

Define mass number

Answer 8

• number of protons

- added to number of neutrons

Question 9

Define isotope

Answer 9

• an atom with the same number of protons but different number of neutrons

Question 10

Who discovered the neutron?

Answer 10

• James Chadwick

Question 11

Who introduced the plum pudding model?

Answer 11

• J.J. Thompson

Question 12

State the formula for relative atomic mass

Answer 12

1/12 mass of one atom of 12C

Question 13

State the formula for relative molecular mass

Answer 13

1/12 mass of one atom of 12C

Question 14

Give the key condition of the mass spectrometer

Answer 14

• vacuum

Question 15

List the 4 steps of mass spectrometry

Answer 15

• ionisation
• acceleration
• ion drift
• detection

Question 16

Summarise how electron impact is used to ionise the sample in mass spectrometry

Answer 16

• sample is vaporised
• high energy electrons are fired at it from an electron gun
• this tends to knock off an electron from each particle
• this forms a 1+ ion
• X(g) + e- –> X+(g) + 2e-

Question 17

When is electron impact ionisation used?

Answer 17

• with elements

- with low Mr compounds

Question 18

When is electrospray ionisation used?

Answer 18

• with high Mr compounds

Question 19

Summarise how electrospray ionisation is used in mass spectrometry

Answer 19

• sample is dissolved in volatile solvent (i.e. water or methanol)
• injected through a fine needle as a spray into the ionisation chamber
• very high voltage is applied to the end of the needle where the spray emerges (the needle is positively charged)
• the particles gain a proton and become ions as a fine mist
• X(g) + H+(g) –> XH+(g)
• solvent evaporates leaving 1+ ions

Question 20

How are the ions accelerated in the mass spectrometer?

Answer 20

• they’re accelerated using an electric field

- so all the ions have the same kinetic energy

Question 21

Summarise ion drift in mass spectrometry

Answer 21

• the ions enter the flight tube (length d)
• ions with different masses have a different time of flight
• the lighter ions travel faster and take less time to reach the detector
• t = d _/m/2KB

Question 22

Summarise detection in mass spectrometry

Answer 22

• the detector is a negatively charged plate
• when the positive ions hit the plate, they are reduced and gain an electron
• this causes a flow of current
• the mass of the ions hitting the detector can be calculated due to their time of flight
• the mass spectrum produced by a computer shows the number of particles (abundance) of each mass that hit the detector

Question 23

Define ionisation energy

Answer 23

• The energy required to remove 1 mole of electrons from 1 mole of atoms in the gaseous state

Question 24

What are the units for ionisation energy

Answer 24

kJ/mol

Question 25

Why is the first ionisation energy of sulfur less than that of phosphorus?

Answer 25

• Phosphorus has one electron in each 3P orbital
• Sulphur has two electrons in one of these orbitals
• So the repulsion of the electrons makes it easier to remove one of them, despite the increased nuclear charge

Question 26

What’s the trend in ionisation energies going down a group?

Answer 26

• Decreases

- Because more shielding so less attraction to the nucleus

Question 27

Why is the second ionisation energy of any atom larger than the first ionisation energy?

Answer 27

• Because the seconds ionisation energy is removing an electron from a 1+ ion
• So the energy has to also overcome the attractive forces between the ion and the electron